1. Field of the Invention
The invention pertains to a control valve for a hydraulic motor used for lifting loads, in particular a directional control valve of the type having a control slide installed in a longitudinal bore of a valve body with freedom to move axially to control the flow of hydraulic oil between two working connection bores, a pump connection bore, and a tank connection bore.
2. Description of the Related Art
Hydraulic motors are used in cranes, for example, to drive hydraulic winches. A winch of this type can lift and lower loads.
A hydraulic directional control valve which makes it possible to control a hydraulic motor independently of the load pressure is known from DE 39 41 802 A1. A directional control valve which can be used for the same purpose is known from DE 41 36 991 C2.
Because of the way in which hydraulic motors, usually designed as axial piston machines or more rarely as radial piston machines, work, it is unavoidable for design reasons that the delivery stream does not flow uniformly but rather fluctuates cyclically; that is, it pulses, as mentioned, for example, on pages 31 and 353-354 of the book by H. Ebertshäuser entitled “Fluidtechnik von A bis Z” [“Fluid Engineering from A to Z”], Vereinigte Fachverlage Krausskopf/Ingenieur-Digest, 1st edition, 1989. This leads unavoidably to torque fluctuations, which become especially bothersome at low rpm's. When a load, initially at rest, is lifted, the transition to the moving state occurs with more or less of a jerk. The same effect occurs when the load has almost reached its intended final position. Pulsations in the movement of the load are very troublesome in this situation also.